Moist heat packs for applying heat to localized body areas for therapeutic purposes, are normally constructed of heavy fabric, such as cotton canvas duck, and are filled with absorbent material which retains and provides moist heat for at least thirty minutes. In the past, moist heat packs have been heated with hot water in a stainless steel tank, since stainless steel may be effectively cleaned when the water is drained from the tank. However, stainless steel is a relatively poor heat conductor, and since the tanks are usually deep (11 inches-30 inches in depth), internally mounted electric dishwasher type calrod, electrical resistance heaters have been used to heat the volume of water within the tank. In a dishwasher with detergent and water circulation under pressure, calrod electrical resistance heaters are effective heating elements, but in a moist heat unit with standing water, dirt and sediment collects on the calrod electrical resistance heaters and they are difficult to access for cleaning. As dirt builds up, the calrod electrical resistance heater corrode, burn out and must be replaced. Also, if water is allowed to evaporate and the calrod electrical resistance heater is energized for a period of time in a dry tank, it is likely to fail.
Past attempts to use silicone rubber type electric resistance heating pads to the bottom of a stainless steel tank for a moist heat unit have failed, for at the minimum wattage needed to effectively penetrate the large area of stainless steel and heat and hold heat dissipating water at a temperature in the range of 150-180.degree. F., the heaters delaminated from the tank. Even at wattages as low as 31/2 watts per square inch, delamination occurred.